1. Field of the Invention
Embodiments of the invention generally relate to an optical system calibration system and method. More particularly, embodiments of the invention are directed to a system and method for calibrating an ophthalmic diagnostic instrument. Most particularly, an optical slit plane calibration apparatus and method are disclosed.
2. Description of Related Art
Eye models provide a valuable tool for assessing the optical, physical and biophysical characteristics of human eyes and those of other species. In order to construct accurate eye models, the shapes and locations of the various ocular surfaces as well as their functions and effects need to be known with a high degree of precision. Alignment has always been a serious issue in creating eye models from image data. Misalignment errors directly result in errors in both estimates of optical power and details about the reconstructed ocular surfaces.
Bausch & Lomb Incorporated (Rochester, N.Y.) is an example of one company that has traditionally used a diagnostic slit beam to illuminate the eye. The slit beam is projected from about a 45 degree angle with respect to the instrument axis. A CCD camera is positioned along the axis. A subject's eye is aligned with the instrument axis and the subject's cornea is positioned at an imaging plane of the camera. The slit beam is scanned in a step-wise manner across the cornea for each eye measurement. The camera captures an image at each step of the slit beam position. By examining a series of these digital images of the slit beam illumination on various surfaces of the eye, three-dimensional models of the eye's surfaces can be obtained. In order to obtain accurate modeling results, it is desirable to obtain good characterization of the slit beam profile and accurate calibration of slit image displacement as measured on the camera detector. In particular, the relationship between the measured parameters and the physical displacement of the scattering surfaces of the eye that are intersected with the slit beam are determined at various locations along the slit beam optical path. The characterization and calibration just referred to will herein be referred to as slit plane calibration.
Slit plane calibration is typically performed with the use of an optical slit projection source, a camera/detector, and a test plate, having known position and orientation coordinates in a plane. The test plate has a scattering surface that is typically flat and painted black to reduce the amount of diffusely reflected light. This test plate is placed close to the imaging plane of the camera and the scattering surface is aligned normal to the instrument axis. The scattering surface is then positioned precisely at a number of axial positions and slit images are captured for the scan sequence at each axial position. Using triangulation techniques, one can analyze where the slit beam impinges on the test plate for a given calibration set-up and calibrate the slit beam width and optical path for that set-up from the known and measured data. This is repeated for additional set-ups to obtain full system calibration.
There are several drawbacks associated with the aforementioned calibration technique and apparatus. The precise placement of the test plates presents challenges in regard to mechanical alignment repeatability and mechanical alignment accuracy. Traditional alignment plates/fixtures can unknowingly be bent or otherwise deformed thus affecting alignment accuracy. The need for multiple physical alignment measurements and sets of images increases the opportunity for error. In addition, it is often necessary to position extra optical occluders along the path of the slit beam to reduce illumination intensity.
In view of the foregoing difficulties encountered with traditional mechanical alignment apparatus and techniques, the inventors have recognized a need for an apparatus and method that simplifies the calibration measurement, improves measurement accuracy and repeatability, is less time consuming and more technically efficient, is integratable with other diagnostic equipment, and offers other benefits and improvements over current apparatus and techniques that will be appreciated by persons skilled in the art.